During the manufacture of integrated circuits, it is important to be able to detect and isolate very small defects caused by leakages, latch-ups, and other problems. Far field photon emission microscopy has been used in the past to detect photon emissions of very low energy from integrated circuits, thereby helping to isolate these types of defects. Although the sensitivity of far field photon emission microscopy is very high, its spatial resolution, being about 0.5 microns, is inadequate to detect many defects, given the fact that integrated circuits are becoming increasingly small and are already in the low sub-micron range. Thus, it is very difficult to obtain adequate resolution beyond 0.5 microns using far field photon emission microscopy.
The ability to improve resolution using far field photon emission microscopy is limited because resolution is dependent on the wavelength of the emitted light and the numerical aperture of the microscope. The resolution of far field photon emission microscopy can only be improved two ways--either by detecting shorter wavelength photons or by increasing the numerical aperture of the microscope. However, most photons emitted by integrated circuit defects have fixed wavelengths. To detect the shorter wavelengths, the integrated circuits can be coated by special materials such as rare earth chelates capable of emitting short wavelength photons, as discussed in Jerry M. Soden, et al., "IC Failure Analysis: Techniques and Tools for Quality and Reliability Improvement," Proceedings of the IEEE, Vol. 81, No. 5, May 1993, p. 707. However, this kind of material poses radiation problems and is limited in use. In addition, the numerical aperture of a camera is limited by the physical size and focal length of the lens, making it impractical and difficult to increase the numerical aperture to the desirable range.
Laser tips as a source of light have been used in the past for far field collection of light reflected off of the surface of integrated circuits, as discussed in T. D. Harris, et al., "Super-Resolution Imaging Spectroscopy," Applied Spectroscopy, Vol. 48, No. 1, January 1994, p.19A. The light path in such devices may be reversed to perform collection mode near field scanning microscopy. This technique merely collects reflected light, giving a topographical image of the sample. It does not detect defects located below the surface of the integrated circuit.
Integrated circuits have also been analyzed in the past using a multi-step process as described in KLA Instruments Corporation, San Jose, Calif., 1620 EMMI (Emission Microscope For Multilayer Inspection) Operator's Manual, Revision A, June 1990. In one step of this process, the integrated circuit is energized or biased using the proper electrical stimulus, and emitted photons are captured by a conventional lens using far field techniques. In another step, light is shined on the surface of the integrated circuit and the reflected light is captured by a camera. In the final step, the two sets of data are combined. However, the resolution available using this technology is insufficient, being only about 0.5 microns.